| A new mechanism—a macrosegregation model—has been established to explain sedimentation occurring in solid-plus-liquid mixtures (α + L or β + L) in the Pb-Sn system. This solid-plus-liquid mixture resembles a liquid-phase-sintered structure.; Macrosegregation has been observed in a series of Pb-Sn alloys isothermally treated in a liquid region or an α + L (or β + L) two-phase region. These experiments indicate macrosegregation may form during both melting and liquid phase sintering. Depending on the Pb-Sn alloy composition, the (Pb-rich α)/(Sn-rich β) solid phase settles/floats to the bottom/top of the crucible as the liquid generated by the eutectic reaction reaches an amount required for phase separation. This settling/floating persists until such time as a skeletal solid structure (mushy zone) is formed at the crucible bottom/top. The initial settling/floating can give rise to macrosegregation. Elimination of macrosegregation requires substantial time and is accompanied by a reduction in length of the mushy zone, leading to apparent sedimentation of this zone. For melting, the mushy zone completely disappears as a result of solid dissolution; for liquid phase sintering, a mushy zone remains at equilibrium. We argue that this mushy zone sedimentation results from the compositional readjustments accompanying macrosegregation elimination. On this basis, it appears that many observations of sedimentation in liquid phase sintered structures are due to these readjustments, which are shown to be diffusion-controlled. |
